Many systems used in laser technology, optics, optoelectronics and electronics have to switch load paths at high voltages. The load has to be brought into a low-ohmic state within a very short period of time, preferably less than 10 ns, so that high currents can be fed in or capacitances can be transferred quickly and efficiently. Examples are the gas discharge channel of a transversally electrically excited gas discharge laser, for example, a CO2 laser, used for generating light pulses having a small width, also the control of Pockels cells for light modulation and devices for controlling ion flight paths in time-of-flight mass spectrometers. The switching operations have to take place very quickly and repetition rates of up to several kHz have to be possible. Thus, there is a great need for a sturdy high-power switching module that has a long service life and that allows fast feeding of currents or charging and discharging of capacitors over a wide operating voltage range.
When it comes to using such a high-power switching module in a gas discharge laser, the following performance profile, for example, is desirable:
Switching voltage5 to 25 kVSafe reverse voltage15% above the rated switching voltageSwitching time5-15 nsReverse recovery timemax. 5 μsTime jitter switching operation0.6 to max. 1.5 nsRepetition frequency100 to 2500 ppsPeak current (>250 ns)1800 to 5500 AReverse current (>200 ns)40% of the peak currentRate of current rise120-500 A/ns
High-power switching modules typically work with a wide array of active elements. In the case of thyratrons filled with hydrogen or deuterium, short switching times at high voltages and currents are achieved. Problematic aspects are the very large physical dimensions and the need for constant heating, which calls for preheating time. Other drawbacks are cathode fatigue and their finite service life due to hydrogen loss.
In high-power switching modules with semiconductor switching systems that are connected on the primary side, the switching operation is carried out at voltages that can be safely controlled with the currently available semiconductor elements (1 to 1.5 kV). A high-frequency pulse driver transforms the switching operation into the required high-voltage range of several 10 kV. A disadvantage here is the finite bandwidth of such pulse drivers, the large installation dimensions, the extremely high currents on the primary side and their poor efficiency. Moreover, the HV pulse still has to be adjusted. Therefore, such switching systems cannot be used for all applications.
Furthermore, high-power switching modules with so-called “plasma semiconductor switches” (Drift Step Recovery Diodes—DSRD—or Fast Ionization Dynistors—FID) have recently become available on the market as single-stage and multi-stage semiconductor switches in special versions (see the publication titled “High Power Semiconductor-Based Nano and Subnanosecond Pulse Generator with a Low Delay Time” by I. V. Grekhov et al. in IEEE Transactions on Plasma Science, Vol. 33, No. 4, August 2005, pp. 1240-1244). However, there is not yet any substantiation of the reliability and service life of these semiconductor switches, and this currently still stands in the way of using them in industrial products. Fundamentally, these elements cannot be switched off, as a result of which they are not suitable for a number of applications.
Typically, directly switching power modules with bipolar and unipolar switching transistors as well as IGBTs are the most frequently used. The current and voltage ranges that are under consideration in conjunction with the present invention, however, so far call for the series-connection and parallel-connection of several hundred elements. Consequently, in order to compensate for element tolerances, symmetrizing networks are necessary. Above about 300 elements, however, the properties of these symmetrizing networks (limitation of the dynamics, tolerances, power loss) mean that the properties of the semiconductor switches are not fully utilized. The switching speed or current rise speed of a switching module structured in this manner can no longer be increased, the power loss of such systems reaches uneconomical levels.
With typical high-power switching modules, sufficiently high voltages in the range of several 10 kV and switching times of less than 20 ns are only achieved by serially connecting many individual switches (see U.S. Pat. No. 4,425,518). However, the individual switches have to be actuated time-synchronously to the greatest extent possible, as is done in DE 36 30 775 C2 by a stepped transformer. In order to increase the permissible current, a matrix arrangement comprising many circuit breakers is also known (see DE 696 29 175 T2). They can only reliably control currents up to a few 100 A. for purposes of achieving a uniform distribution of the voltage among the individual stages in the case of somewhat different active elements operating at high voltages in the limit ranges of the elements, DD 234 974 A1 describes connecting a chain of resistors in parallel to the switching stages. DE 198 25 056 C1 describes a circuit arrangement for feeding electric energy into the plasma of a glow discharge with a specially interconnected p-type conducting and n-type conducting MOSFET output stage as the driver for a high-power IGBT, so that the power of unipolar or bipolar pulsed plasmas can be increased.
DE 36 30 775 C2 describes a high-power switching module in which several switching stages each have a MOSFET as the semiconductor switch. All of the semiconductor switches are series-connected with their anode and cathode terminals. A control network for the semiconductor switch is provided in each switching stage and this control network is located between the control terminal and a pulse driver equipped with a primary winding and with a number of secondary windings corresponding to the number of switching stages provided. Without this control network, the switching duration of the known high-power switching module is predefined by the voltage-time frame of the pulse driver and it is in the order of magnitude of 100 ns. In order to achieve slightly longer switching durations, the control network has a control diode that is polarized in the conducting direction relative to the induced control voltage, and it also has a control resistor that is connected between the control terminal and the anode terminal of the semiconductor switch. Consequently, in a typical high-power switching module, the control network serves for purposes of possibly prolonging the switching duration as a function of a permanently set control resistance in each switching stage.
Another high-power switching module is described in DE 195 15 279 C2. In this generic high-power switching module, a snubber capacitor and an additional synchronizing resistor (there as a varistor) are connected in parallel to each semiconductor switch between the anode terminal and the cathode terminal. Snubber networks neutralize interfering oscillations or voltage peaks. The high-frequency voltage peaks that arise from the bouncing of the contacts of the semiconductor switch are short-circuited by a snubber capacitor. If switching pulses are induced into the switching stage by the pulse driver, then these switching pulses reach the control terminal of the semiconductor switch via the control diode, they charge its snubber capacitor and the semiconductor switch switches on. As the pulse amplitude subsides, the control diodes, which are then non-conductive, prevent a charge balancing in the snubber capacitor via the pulse driver, so that the snubber capacitor is only discharged via the parallel-connected control resistor. Therefore, the switching duration can be set by the value of the control resistance.
However, with this high-power switching module as well as with all of the above-mentioned high-power switching modules, there is always a fixed association of each switching stage to an operating point at a fixed switching stage voltage. Consequently, element tolerances and operational fluctuations only allow a switching synchronism of between 3 ns and 5 ns at best. However, if individual switching stages switch on sooner than others because of operating point shifts, responses to temperature changes or interference coupling, this can lead to the destruction of all of the semiconductor switches in the high-power switching module. This equally applies to turn-off operations. If an active turn-off operation is carried out, this necessarily has to be done synchronously, since otherwise the voltage at the switching stage that switches first will rise impermissibly and the switching stage will be destroyed.
The switching synchronism in a high-power switching module is determined by the turn-on and turn-off time constants (turn on delay, turn off delay) of the individual switching stages. These, in turn, are influenced by the element parameters, for example, the snubber capacitance, the switching level, the amplification, the linearity and the positive feedback. Additional influences are also the switch periphery such as the control level, the source resistance, the anode voltage and cathode voltage, the load capacitance and load inductance as well as the responses to temperature changes. With typical high-power switching modules, differences in the switching times of the individual semiconductor switches are compensated for by internal positive feedback effects and limited by external protective circuits. Due to the nevertheless still highly varying turn-on and turn-off time constants of the individual switching stages, however, no switching synchronism can be achieved in the lower ns range with the prior-art high-power switching modules.